Of the elements required for informed consent, which may be difficult to achieve with an online survey?

A researcher wants to conduct a study on stress and depression among young adults (age 18 and older) who are attending college. The researcher plans to recruit participants for the study by sending an email to all students enrolled in an introductory psychology course. The email will contain a link to an anonymous online survey that includes questions about the numbers of days the participant has felt sad in the past week, thought about suicide, and felt overwhelmed.

Of the elements required for informed consent, which may be difficult to achieve with an online survey?